Germanium diodes



Dec. 31, 1957 --H. W OLFSON GERMANIUM DIODES Filed Oct. 12, 1953 PULSERu/v/r I. Inventor wo ,F'S ON B Attorney GERMANIUM DIODES enryc- Wolfson,London,v England, assignor to. InternationaLStandard; ElectricCorporation, New York,

NZ Y.., acorporation of'D'elaware Application-October:12;. 1953',-seriakNoz 38.5 650:

Claims.priority,,application-Great Britain. O'ctober 15; 1952 2:laims..(Cl'.31 7-436)v cat whisker making contact with some part of'the crystalsurface not coveredzbyrthe base-electrode;i.

Theprincipal object of the invention is to improve tlierectificati'onratio of such' crystal diodes or rectifiers by:re'ducing;,th'e' forwardresistance without appreciably reducing the reverse resistance.

This object is achieved according to this inventiorr by providinga'methodof" manufacturing an electric rectifier-"consisting of--acrystalof germanium havi-nga-- cat whisker in contact with its surface,which includes the step of producing beneath the whisker point a surfacecontact film consisting of an alloy of lead or tin with the germanium,by means of an electroforming process in which an electric current ispassed through the cat whisker and crystal.

The invention also provides an electric crystal rectifier made inaccordance with this method.-

The invention further provides an electric crystal rectifier comprisinga crystal of germanium having a base electrode making low resistancecontact with part of the surface of the crystal, a thin contact filmconsisting of an alloy of germanium with lead or tin covering anotherpart of the said surface, and a cat whisker electrode making contactwith the said film.

The invention will be described with reference to the accompanyingdrawing, in which:

Fig. 1 shows a diagram illustrating the method of manufacture of acrystal rectifier according to the invention;

Fig. 2 illustrates the rectifier after the process according to theinvention has been applied; and

Fig. 3 shows a modification of Fig. 1.

In the manufacture of point contact germanium diodes, we found that asthe purity (and therefore the specific resistivity) of the germaniumincreases, while the peak back voltage and reverse resistance increase,the forward resistance (required to be as low as possible) alsoincreases. The rectification ratio of reverse resistance to forwardresistance should be as high as possible and can obviously be improvedby decreasing the forward resistance.

For several reasons we also desire to use as point contact materialtungsten cat whiskers. We have found that the use of platinum contacts,while under certain conditions yielding higher forward currents,invariably has a detrimental lowering effect on reverse resistance.

It is the usual practice to subject a germanium rectifier to anelectroforming process in order to improve its characteristics. In thisprocess electrical forming pulses of current are passed through therectifying contact. An account of this process can be found in thetextbook entitled Crystal Rectifiers by H. C. Forrey and C. A.

2,818,537 Patented Dec. 3 1 11957 2. Whitmer,publi'shetl'ib'yithe-McGi'aw Hill Book Company, Inc., 1948; pages 370'and-37'1.

It is known" that: impurities may be injected into the germaniumcrystal'from the cat whisker-during the=e lectroforming-processto effectcertaimconductivity changes; forexample, toobtain-P or N-typeconductivity, impurities chosen respectively from grou pj IIl and group;V" of thegperiodi'e-table arecomm only used, These are usually 'known'as significant impuritieszi We have investigated the=- eifject' of Einjecting, during the usual electroforming process; elements from groupIV 'ofthe periodic; table, i. e; the group toywhicii germanium'bel'ongs: Itwould seem that as' far as reverse characteristicsareconcerned" such elements would'be non-significan ;"'but that-theymaybe expected to affect the forward condu'ctanceof the rectifier.

As; a result ofi-this= work we have; fbundthat weare ableto improve the,forward* conductance (lowerthe forward" resistance) by',a;;factor' ofup; to Sj'times; in the mannenillustrated iiijFig; 1, whichshows-asectionalview of agermanium rectifier-an-dthe connections to anelectroforming circu-itif The desi'rediresults-are"achieved moresuccessfully; the higher the specific resistivity (the greater the;purity) of the-germanium: 1

Referring to Fig; l, a germanium crystalv slice; 1"; is provided 'in'some suitable" waywith a base; electrode 2 making l'owresistance-contactwitl'rthelower-surface of the slice. To theuppen-surface of tlre -slice fie-applied a thin layer 3 of lead oxide ofhigh purity, free from elements from group III or group V of theperiodic table. The oxide Pb O (also known as red lead or minium) is thepreferred oxide to use. If desired, the layer may be bonded to thesurface of the crystal with a trace of an organic binder such asnitrocellulose, the amount of the binder not being sufiicient to makethe layer non-conducting.

A cat whisker 4, preferably of tungsten, is brought into contact withthe layer 3, and electrical forming pulses are applied in the usual way,for example by means of a circuit consisting of a pulser unit 5providing pulses such as those described at the top of page 371 of thetextbook already referred to, and an adjustable resistor 6, between thebase electrode 2 and the cat whisker 4, as indicated in Fig. l.

The resistor 6 should be adjusted so that the current pulses producedare able to generate sutficient heat at the cat whisker point todecompose the lead oxide layer 3 and to alloy the lead set free with thegermanium at the surface. The rectifier then appears as shown in sectionin Fig. 2, in which a low resistance contact film 7 has been formedbelow the point of the cat whisker 4. This film consists of a thin layerof germanium-lead alloy. The reduction in the forward resistance of therectifier is believed to result principally from the effective increasein the contact area of the cat whisker 4, but the reverse resistance,which is produced mainly by the P-N junction or barrier which existsbelow the surface of the crystal, is not thereby appreciably affected,and so the rectificatton ratio is increased.

It may be added that we believe that the oxygen set free by thedecomposition of the lead oxide prevents a deterioration of the reverseresistance characteristic of the rectifier by the heat necessary fordecomposition.

An alternative method of carrying out the invention is illustrated inFig. 3. In this case no layer of lead oxide is provided, but instead,the cat whisker 4 is provided at the pointed end with a plated coating 8of pure lead, produced for example, by electrode deposition using asulphamate electrolyte. On applying the electroforming pulses as in Fig.1, some of the lead is transferred to, and alloyed with, the surface ofthe germanium, and a result similar to Fig. 2 is obtained. It isessential that the lead coating 8 should be free from significantimpurities unless it is desired at the same time to bring about changesin the reverse characteristic of the rectifier.

By the method of Fig. 3, the same desirable increase in forward currentresults, but it has been found that more skill is necessary in carryingout the electroforming process if the reverse resistance is not to bereduced.

It may be added that tin oxide may be used instead of lead oxide for thelayer 3 of Fig. 1, and in that case the contact film 7 of Fig. 2 willconsist of an alloy of germanium and tin. However, a smaller improvementin rectification ratio is to be expected, and so lead oxide ispreferred. Likewise, in Fig. 3, the cat whisker 4 may be plated with tininstead of lead.

The improvement in rectification ratio obtained by the method accordingto the invention depends on the specific resistivity of the germaniumemployed. Thus for example, using germanium of low specific resistivity,for example 2 or 3 ohm/cm., the forward current for a conventionalrectifier using a tungsten cat whisker may be of the order of 5milliarnps. at 1 volt. When according to the invention a germanium-leadalloy contact film is formed, the forward current will increase tobetween 7 and 10 milliarnps., for example, without appreciably affectingthe reverse current. If, for example, the specific resistivity of thegermanium is in the range 10 to ohm/cm., then the forward current whichin a conventional rectifier may be 2 to 4 milliarnps., is increased tofrom 7 to 20 milliarnps. These results may be expected whether themethod of Fig. 1 or Fig. 3 is used. However, if tin is used instead oflead, a forward current of about 2 to 4 milliarnps. is increased to from4 to 10 milliarnps., which is less than the increase when lead is used.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What I claim is:

1. An electric crystal rectifier comprising a semiconductor body withcontacts applied to opposite surfaces thereof, comprising a thin contactfilm of an alloy of said semiconductor with a metal chosen from thegroup consisting of lead and tin covering a part of one of said surfacesand a cat whisker electrode in contact with said film.

2. An electric crystal rectifier comprising a crystal of germaniumhaving a base electrode making low resistance contact with part of thesurface of the crystal, a thin contact film of an alloy of germaniumwith one metal of the group consisting of lead and tin covering anotherpart of the said surface, and a cat whisker electrode making contactwith the said film.

References Cited in the file of this patent UNITED STATES PATENTS2,438,110 Brattain Mar. 23, 1948 2,504,628 Benzer Apr. 18, 19502,514,879 Lark-Horovitz et a1 July 11, 1950 2,530,110 Woodyard Nov. 14,1950 2,623,102 Shockley Dec. 23, 1952 2,671,156 Douglas et a]. Mar. 2,1954

1. AN ELECTRIC CRYSTAL RECTIFIER COMPRISING A SEMICONDUCTOR BODY WITHCONTACTS APPLIED TO OPPOSITE SURFACES THEREOF, COMPRISING A THIN CONTACTFILM OF AN ALLOY OF SAID SEMICONDUCTOR WITH A METAL CHOSEN FROM THEGROUP CONSISTING OF LEAD AND TIN COVERING A PART OF ONE OF SAID